Figure 1.

Bone defects coalesce anteriorly in meox1 ^cho mutant zebrafish. (A,B) Lateral views of whole‐mount alizarin red‐stained 32‐mm standard length adult siblings and homozygotes reveal that bone defects within meox1 ^cho homozygotes are confined to the axial skeleton. The boxed regions are magnified in (A′) and (B′). A green arrow indicates the location of the pectoral girdle. (A′,B′) The exoccipital foramen is readily discerned in siblings (A′, arrowhead, n = 6/6) and is not evident in whole‐mount meox1 ^cho homozygous mutants (B′, arrowhead, n = 6/6). Vertebral fusion is evident in vertebræ across neural arches (B′, arrows, n = 6/6). (C) Lateral maximum intensity Z‐projection of a chemically isolated vertebra shows the morphology typical of a vertebra from the anterior region of the wild‐type zebrafish spine. (D) Lateral view of a chemically isolated vertebra involved in an apparent anterior fusion in a meox1 ^cho adult mutant. Fusion occurred between neural arches (arrows); however, only one vertebral centrum was incorporated. na, neural arch; ns, neural spine. Scale bars: 1 cm (A,B); 500 μm (C,D).